Processing of oxide superconductors

ABSTRACT

A method for preparing a BSCCO-2223 oxide superconducting article includes annealing an oxide superconductor article comprised of BSCCO-2223 oxide superconductor at a temperature selected from the range of about 500° C.≦T≦787° C. and an annealing atmosphere having an oxygen pressure selected from within the region having a lower bound defied by the equation, P O2 (lower)≧3.5×10 10 exp(−32,000/T+273) and an upper bound defined by the equation, P O2 (upper)≦1.1×10 12 exp(−32,000/T+273). The article is annealed for a time sufficient to provide at least a 10% increase in critical current density as compared to the critical current density of the pre-anneal oxide superconductor article. An oxide superconductor having the formula Bi 2−y Pb y Sr 2 Ca 2 Cu 3 O 10+x , where 0≦x≦1.5 and where 0≦y≦0.6 is obtained, the oxide superconductor characterized by a critical transition temperature of greater than 111.0 K, as determined by four point probe method.

[0001] The application is a continuation-in-part application ofco-pending application U.S. Ser. No. 08/041,822 filed Apr. 1, 1993, alsoentitled “Improved Processing for Oxide Superconductors”.

FIELD OF THE INVENTION

[0002] The present invention relates to high-performance oxidesuperconductors and oxide superconductor composites. The presentinvention further relates to a method for healing defects introducedinto the oxide superconductor phase during processing thereby improvingsuperconducting properties. The present invention also relates to theprocessing of high performance bismuth-strontium-calcium-copper oxidesuperconductors and oxide superconductor composites and a method forimproving the critical transition temperature (T_(c)) and criticalcurrent density (J_(c)) of these oxide superconductors.

BACKGROUND OF THE INVENTION

[0003] Oxide superconductors of the rare earth-barium-copper-oxidefamily (YBCO), bismuth(lead)-strontium-calcium-copper-oxide family((Bi,Pb)SCCO) and thallium-barium-calcium-copper-oxide family (TBCCO)form plate-like and highly anisotropic superconducting oxide grains.Because of their plate-like morphology, the oxide grains can be orientedby mechanical strain. Mechanical deformation has been used to inducegrain alignment of the oxide superconductor c-axis perpendicular to theplane or direction of elongation. The degree of alignment of the oxidesuperconductor grains is a major factor in the high critical currentdensities (J_(c)) obtained in articles prepared from these materials.

[0004] Known processing methods for obtaining textured oxidesuperconductor composite articles include an iterative process ofalternating heating and deformation steps. The heat treatment is used topromote reaction-induced texture of the oxide superconductor in whichthe anisotropic growth of the superconducting grains is enhanced. Eachdeformation provides an incremental improvement in the orientation ofthe oxide grains. Additional heat treatment intermediate with orsubsequent to deformation is also required to form the correct oxidesuperconductor phase, promote good grain interconnectivity and achieveproper oxygenation.

[0005] Processing long lengths of oxide superconductor is particularlydifficult because deformation leads to microcracking and other defectswhich may not be healed in the subsequent heat treatment. Cracks thatoccur perpendicular to the direction of current flow limit theperformance of the superconductor. In order to optimize the currentcarrying capability of the oxide superconductor, it is necessary to healany microcracks that are formed during processing of the oxidesuperconductor or superconducting composite.

[0006] Liquid phases in co-existence with solid oxide phases have beenused in processing of oxide superconductors. One type of partialmelting, known as peritectic decomposition, takes advantage of liquidphases which form at peritectic points of the phase diagram containingthe oxide superconductor. During peritectic decomposition, the oxidesuperconductor remains a solid until the peritectic temperature isreached, at which point the oxide superconductor decomposes into aliquid phase and a new solid phase. The peritectic decompositions ofBi₂Sr₂CaCu₂O_(8+x), (BSCCO-2212, where 0≦x≦1.5), into an alkaline earthoxide and a liquid phase and of YBa₂Cu₃O_(7−δ) (YBCO-123, where 0≦δ≦1.0)into Y₂BaCuO₅ and a liquid phase are well known.

[0007] Peritectic decomposition of an oxide superconductor and thereformation of the oxide superconductor from the liquid+solid phase isthe basis for melt textured growth of YBCO-123 and BSCCO-2212. Forexample, Kase et al. in IEEE Trans. Mag. 27(2), 1254 (March 1991) reportobtaining highly textured BSCCO-2212 by slowly cooling through theperitectic point. This process necessarily involves total decompositionof the desired 2212 phase into an alkaline earth oxide and a liquidphase.

[0008] It is also recognized that an oxide superconductor itself canco-exist with a liquid phase under suitable processing conditions. Thismay occur by solid solution melting, eutectic melting or by formation ofnon-equilibrium liquids.

[0009] Solid solution melting may arise in a phase system, in which theoxide superconductor is a solid solution. As the temperature (or someother controlling parameter) of the system increases (or decreases), theoxide superconductor phase changes from a solid oxide phase to a liquidplus oxide superconductor partial melt (this happens at the solidus). Afurther increase in temperature (or some other controlling parameter)affords the complete melting of the oxide superconductor (this happensat the liquidus).

[0010] A phase diagram containing a eutectic point may provide an oxidesuperconductor partial melt, known as eutectic melting, when the overallcomposition is chosen so as to be slightly off stoichiometry. As thetemperature (or some other controlling parameter) of the systemincreases (or decreases), the mixed phase of oxidesuperconductor-plus-non-superconducting oxide (solid₁+solid₂) changes toa liquid-plus-oxide superconductor partial melt (solid₁+liquid).

[0011] Non-equilibrium liquids may also promote partial melting in oxidesuperconductor systems. A non-equilibrium liquid is established throughthe relatively rapid heating of a mixture of oxides to a temperatureabove the eutectic melting point of local stoichiometries present in theheterogeneous mixture of phases. As the oxides form the desired oxidesuperconductor, the solid and liquid phases can co-exist, if onlytemporarily.

[0012] Partial melting of (Bi,Pb)₂Sr₂Ca₂Cu₃O_(10+x) ((Bi,Pb)SCCO-2223,where 023 x≦1.5) and (Bi)₂Sr₂Ca₁Cu₂O_(10+x) ((Bi)SCCO-2223, where0≦x≦1.5), collectively BSCCO-2223, at temperatures above 870° C. in airhas been reported; see, for example, Kobayashi et al. Jap. J. Appl.Phys. 28, L722-L744 (1989), Hatano et al. Ibid. 27(11), L2055 (Nov.1988), Luo et al. Appl. Super. 1, 101-107, (1993), Aota et al. Jap.J.Appl. Phys. 28, L2196-L2199 (1989) and Luo et al. J. Appl. Phys. 72,2385-2389 (1992). The exact mechanism of partial melting of BSCCO-2223has not been definitively established.

[0013] Guo et al. in Appl. Supercond. 1(½), 25 (January 1993) havedescribed a phase formation-decomposition-reformation (PFDR) process, inwhich a pressed sample of (Bi,Pb)SCCO-2223 is heated above its liquidusto decompose the 2223 phase, followed by a heat treatment at atemperature below the solidus. The sample was subsequently pressed againand reannealed. The final anneal of the PFDR process includes a standardsingle step heat treatment in which there is no melting.

[0014] The “high T_(c)” oxide superconductorBi_(2−y)Pb_(y)Sr₂Ca₂Cu₃O_(10+x), where 0≦x≦1.5 and 0≦y≦0.6 (BSCCO-2223and (Bi,Pb)SCCO-2223, hereinafter referred to as “BSCCO-2223 ” toindicate both lead-doped and undoped compositions), is particularlydesirable because of its high critical transition temperature (T_(c)˜110K) and high critical current (I_(c), J_(c)). The superconducting artconstantly seeks to improve electrical properties, such as, criticalcurrent density and critical transition temperature.

[0015] Partial melting in the processing of oxide superconductors hasbeen used either to increase the yield of the BSCCO-2223 phase or toimprove the contiguity and texturing of the oxide superconductor grains.The issue of healing defects, such as microcracks, which develop duringprocessing of the oxide superconductor, has not been addressed. Further,the prior art has not addressed the possibility of using a two-stepprocess where the oxide superconductor is stable in both steps for thehealing of cracks and defects.

[0016] Wang et al. (“Advances in Superconductivity”, Springer-Verlag,New York, Editors: Y. Bando and H. Yamauchi, pp. 291-294 (1993)) reportan increase in T_(c) by carrying out a post-anneal step at 790° C. atreduced total pressures. Wang et al. observed T_(c) by DC magnetizationof 115 K and T_(c, zero) of 111 K by resistivity measurement. Thetechnique used by Wang et al. (vacuum encapsulation at 10⁻⁴ Torr ofoxide superconductor pellets, followed by annealing at 790° C.) does notpermit determination of the oxygen pressure of the system. Theencapsulated pellets reach an equilibrium oxygen pressure within thecapsule by releasing oxygen. The pellet volume/capsule volume plays animportant role in determining the final equilibrium oxygen pressure.

[0017] Critical transition temperatures (determined by magnetization) ashigh as 117 K have been reported for multiphase materials containingBSCCO-2223. Fisher et al. (Physica C 160, 466 (1990)) reported a T_(c)of 115 K (determined by magnetization) with the substitution of lead andantimony in the BSCCO-2223 system. A non-reproducible T_(c) as high as130 K was reported by Hongbo et al. (Solid State Comm. 69, 867 (1989)).

[0018] While reports of high transition temperatures by magnetizationstudies are of interest, they can sometimes be misleading. Thetransition curves obtained by magnetization are “soft”, makingextrapolation to zero resistivity highly subjective. Further, othereffects, such as semiconductor to metallic transitions, can mimiccritical temperature transition behavior. It is therefore desirable torely on bulk resistivity measurements for determining temperature atzero resistivity (T_(c, zero)).

[0019] Idemoto et al. (Physica C 181, 171-178 (1991)) has investigatedthe oxygen content and copper and bismuth valances of BSCCO-2223 under arange of conditions, including temperatures in the range of 500° C. to850° C. and oxygen pressures in the range of 0.005 to 0.20 atm. Thesamples were observed by means of a microbalance under changingtemperatures conditions at constant oxygen pressures. Because thesamples do not reach equilibrium during the observation period, it isdifficult to determine the exact processing conditions experienced bythe samples. No investigation of the effect of reported conditions onelectrical properties is reported.

[0020] None of the previous research has indicated the desirability ofpost-annealing the BSCCO-2223 phase at low temperatures and oxygenpressures to enhance the electric transport properties of the oxidesuperconductor, namely critical current.

[0021] It is the object of the present invention to provide a method forimproving superconducting performance of oxide superconductors andsuperconducting composites by healing cracks and defects formed duringprocessing of oxide superconductors and superconducting composites.

[0022] It is a further object of the invention to prepare oxidesuperconducting articles having significantly less cracks and defectsthan conventionally-processed articles.

[0023] A further object of the present invention is to provide a processto increase the critical current density of BSCCO-2223 by a method whichalso increases its critical transition temperature. It is a furtherobject of the present invention to provide a novel high-T_(c) BSCCO-2223composition having a critical transition temperature greater than 111.0K.

[0024] A feature of the invention is a two-step heat treatment afterwhich no further deformation occurs which introduces a small amount of aliquid phase co-existing with the oxide superconductor phase, and thentransforms the liquid back into the oxide superconductor phase with nodeformation occurring during or after the heat treatment of theinvention. A further feature of the present invention is a lowtemperature, low oxygen pressure anneal of the oxide superconductor.

[0025] An advantage of the invention is the production of highlydefect-free oxide superconductor and superconducting composites whichexhibit superior critical current densities. A further advantage of theinvention is a marked improvement in critical transition temperature andcritical current density as compared to oxide superconductors andsuperconducting composites which are not subjected to the method of theinvention.

SUMMARY OF THE INVENTION

[0026] In one aspect of the present invention, an oxide superconductorarticle containing a desired oxide superconductor phase is exposed to atwo-step heat treatment after deformation of the article, which includes(a) heating the article at a temperature sufficient to partially meltthe article, such that a liquid phase co-exists with the desired oxidesuperconductor phase; and (b) cooling the article to a temperaturesufficient to transform the liquid phase into the desired oxidesuperconductor, with no deformation occurring after the heat treatmentof the invention.

[0027] In another aspect of the invention, an oxide superconductorarticle containing a desired oxide superconductor phase is exposed to atwo-step heat treatment after deformation of the article which includes(a) forming a liquid phase in the oxide superconducting article, suchthat the liquid phase co-exists with the desired oxide superconductorsolid phase; and then (b) transforming the liquid phase into the desiredoxide superconductor, with no deformation occurring after the heattreatment of the invention.

[0028] In preferred embodiments, the liquid phase wets surfaces ofdefects contained within the oxide superconductor article. The defectsare healed upon transformation of the liquid to the desired oxidesuperconductor. The partial melting of step (a) and the transformationof step (b) are effected by selection of appropriate thermodynamic statevariables, for example, temperature, P_(O2), P_(total) and totalcomposition. In principle, deformation may occur during the heattreatment of the present invention up to immediately prior to thecompletion of step (a), providing that the liquid phase is available fora period of time sufficient to wet defect surfaces.

[0029] By “two-step heat treatment” or “heat treatment of theinvention”, as those terms are used herein, it is meant a heat treatmentfor forming an oxide superconductor after which no further deformationoccurs. However, heat treatments for purposes other than those statedherein, such as, for example, oxygenation of the oxide superconductor,are possible. In all cases, not further deformation occurs.

[0030] By “partial melt”, as that term is used herein, it is meant theoxide superconductor article is only partially melted, and that thedesired oxide superconductor is present during melting.

[0031] By “deformation” as that term is used herein, it is meant aprocess which causes a change in the cross-sectional shape of thearticle.

[0032] By “oxide superconductor precursor”, as that term is used herein,it is meant any material that can be converted to an oxidesuperconductor upon application of a suitable heat treatment. Suitableprecursor materials include but are not limited to metal salts, simplemetal oxides, complex mixed metal oxides and intermediate oxidesuperconductors to the desired oxide superconductor.

[0033] By “desired oxide superconductor”, as that term is used herein,it is meant the oxide superconductor which it is desired to ultimatelyprepare. An oxide superconductor is typically the “desired” oxidesuperconductor because of superior electrical properties, such as highT_(c) and/or J_(c). The desired oxide superconductor is typically a highT_(c) member of a particular oxide superconductor family, i.e.,BSCCO-2223, YBCO-123, TBCCO-1212 and TBCCO-1223.

[0034] By “intermediate oxide superconductor”, as that term is usedherein, it is meant an oxide superconductor which is capable of beingconverted into a desired oxide superconductor. However, an intermediateoxide superconductor may have desirable processing properties, whichwarrants its formation initially before final conversion into thedesired oxide superconductor. The formation of an “intermediate oxidesuperconductor” may be desired, particularly during heattreatment/deformation iterations, where the intermediate oxides are moreamenable to texturing than the desired oxide superconductor.

[0035] In yet another aspect of the present invention, a textured oxidesuperconductor article is prepared by subjecting an article containingan oxide superconductor precursor to at least one first heattreatment/deformation iteration. The heat treatment is effective to forma desired oxide superconductor. The resultant oxide superconductor phaseis textured upon application of the first heat treatment/deformationiteration. The article is then exposed to a two-step heat treatment inwhich (a) the article is partially melted, such that a liquid phaseco-exists with the desired textured oxide superconductor phase; and (b)the liquid phase is transformed into the desired oxide superconductor,with no deformation occurring after the heat treatment of the invention.

[0036] In yet another aspect of the present invention, a textured oxidesuperconductor article is prepared by subjecting an article containingan oxide superconductor precursor to at least one first heattreatment/deformation iteration. The heat treatment is effective to forman intermediate oxide superconductor. The intermediate textured oxidesuperconductor phase is formed. The article is then subjected to atleast one second heat treatment/deformation iteration. The heattreatment is effective to form a desired oxide superconductor. Thedesired textured oxide superconductor is formed. The article is thenexposed to a two-step heat treatment in which (a) the article ispartially melted, such that a liquid phase co-exists with the desiredtextured oxide superconductor phase; and (b) the liquid phase istransformed into the desired oxide superconductor, with no deformationoccurring after the heat treatment of the invention.

[0037] In preferred embodiments, the intermediate oxide superconductoris BSCCO-2212 or (Bi,Pb)SCCO-2212 because it is readily textured by theheat treatment/deformation iterations. The intermediate oxidesuperconductor is then converted to a desired oxide superconductingphase, typically BSCCO-2223 or (Bi,Pb)SCCO-2223. The partial melting ofstep (a) may be carried out at a temperature in the range of 820-835° C.at 0.075 atm O₂. The transformation of the liquid in step (b) may becarried out at a temperature in the range of 790-820° C. at 0.075 atmO₂. In other preferred embodiments, the desired oxide superconductor,may be YBCO-123, Y₂Ba₄Cu₇O_(14−δ), (YBCO-247),(Tl,Pb)₁Ba₂Ca₁Cu₂O_(6.0±y) (TBCCO-1212) or (Tl,Pb)₁Ba₂Ca₂Cu₃O_(8.0±y)(TBCCO-1223), where 0≦δ≦61.0 and y ranges up to 0.5. The statedstoichiometries are all approximate and intentional or unintentionalvariations in composition are contemplated within the scope of theinvention.

[0038] In other preferred embodiments, the liquid phase is formed in therange of 0.1-30 vol %. In yet other preferred embodiments, the heattreatment of the first and second heat treatment/deformation iterationspartially melts the oxide superconductor article.

[0039] In yet another aspect of the invention, an oxide superconductorarticle is exposed to a two-step heat treatment after a deformationstep, which includes (a) heating the article at a temperaturesubstantially in the range of 810-860° C. for a period of timesubstantially in the range of 0.1 to 300 hours at a P_(O2) substantiallyin the range of 0.001-1.0 atm; and (b) cooling the article to atemperature substantially in the range of 780-845° C. for a period oftime substantially in the range of 1 to 300 hours at a P_(O2)substantially in the range of 0.001-1.0 atm, with no deformationoccurring after the heat treatment of the present invention.

[0040] In yet another aspect of the present invention, an oxidesuperconductor article containing a desired oxide superconductor phaseis exposed to a two-step heat treatment after a deformation step, whichincludes (a) subjecting the article to an oxygen partial pressuresufficient to partially melt the oxide superconducting article, suchthat a liquid phase co-exists with the desired oxide superconductor; and(b) raising to an oxygen partial pressure sufficient to transform theliquid phase into the desired oxide superconductor.

[0041] Yet another aspect of the present invention provides for amultifilamentary oxide superconductor composite containing a pluralityof oxide superconductor filaments contained within a matrix materialwhich has been subjected to the two-step heat treatment of theinvention.

[0042] In yet another aspect of the invention, a multifilamentary oxidesuperconductor composite contains a plurality of oxide superconductorfilaments contained within a matrix material, the composite having aJ_(c) of at least 14×10³ A/cm² at 77K, self field, as measured over alength of at least 50 m.

[0043] The present invention provides oxide superconductors whichexhibit marked improvement in critical current density (J_(c)) oversamples processed in an otherwise similar manner, lacking only thetwo-step heat treatment of the present invention.

[0044] In yet another aspect of the present invention, a BSCCO-2223oxide superconducting article is prepared by providing an oxidesuperconductor article including BSCCO-2223 oxide superconductor, andannealing the superconducting article at a temperature selected from therange of about 500° C.≦T≦787° C. and an annealing atmosphere having anoxygen pressure selected from within the region having a lower bounddefined by the equation, P_(O2)(lower)≧3.5×10¹⁰exp(−32,000/T+273) and anupper bound defined by the equation,P_(O2)(upper)≦1.1×10¹²exp(−32,000/T+273). The sample is annealed for atime sufficient to provide at least a 10% increase in critical currentdensity as compared to the critical current density of the pre-annealoxide superconductor article.

[0045] In yet another aspect of the invention, a BSCCO-2223 oxidesuperconducting article is prepared by providing an oxide superconductorarticle including BSCCO-2223 oxide superconductor, and annealing thesuperconducting article at a temperature selected from the range ofabout 500° C.≦T≦760° C. and an annealing atmosphere having an oxygenpressure selected from within the region having a lower bound defined bythe equation, P_(O2)(lower)≧8.5×10¹⁰exp(−32,000/T+273) and an upperbound defined by the equation,P_(O2)(upper)≦2.62×10¹¹exp(−32,000/T+273). The sample is annealed for atime sufficient to provide at least a 10% increase in critical currentdensity as compared to the critical current density of the pre-annealoxide superconductor article.

[0046] In yet another aspect of the invention, a BSCCO-2223 oxidesuperconductor article is prepared by exposing the article including atleast BSCCO-2223 to a heat treatment after deformation of the article,including (a) heating the article at a temperature substantially in therange of 815-860° C. for a period of time substantially in the range of0.1 to 300 hours at a P_(O2) substantially in the range of 0.001-1.0atm; and (b) subjecting the article to a temperature substantially inthe range of 790-845° C. for a period of time substantially in the rangeof 1 to 300 hours at a P_(O2) substantially in the range of 0.01-1.0atm, with no deformation occurring after the heat treatment. Thesuperconducting article is then annealed at a temperature selected fromthe range of about 500° C.≦T≦787° C. and an annealing atmosphere havingan oxygen pressure selected from within the region having a lower bounddefined by the equation, P_(O2)(lower)≧3.5×10¹⁰exp(−32,000/T+273) and anupper bound defined by the equation,P_(O2)(upper)≦1.1×10¹²exp(−32,000/T+273).

[0047] By “anneal of the present invention”, it is meant a low pressure,low temperature heat treatment under equilibrium conditions during whichno further formation of the desired oxide superconducting phase occurs;however, the internal chemistry of the oxide superconductor (i.e.;oxygen stoichiometry) and grain growth of the existing oxidesuperconductor phase may be affected.

[0048] In preferred embodiments, the annealing atmosphere issubstantially at a pressure of one atmosphere and oxygen pressure isobtained by controlling the oxygen concentration in the annealingatmosphere. The annealing atmosphere may additionally contain an inertgas selected from the group consisting of argon, nitrogen and helium.The anneal is carried out at an oxygen pressure substantially in therange of 7.5×10⁻² atm to 1×10⁻⁸ atm O₂. The annealing of the inventionis preferably the final annealing to which the superconducting articleis subjected.

[0049] In other preferred embodiments, the anneal is carried out at athe temperature in the range of 770 to 787° C. and an oxygen pressure inthe range of 0.017 to 0.085 atm; The anneal is carried out at atemperature in the range of 750 to 770° C. and an oxygen pressure in therange of 0.0009 to 0.052 atm; the method of claim 1, 2 or 3, wherein theanneal is carried out at a temperature in the range of 730 to 750° C.and an oxygen pressure in the range of 0.005 to 0.029 atm; the anneal iscarried out at a temperature in the range of 690 to 730° C. and anoxygen pressure in the range of 0.0001-0.015 atm; the anneal is carriedout at a temperature in the range of 740 to 760° C. and an oxygenpressure in the range of 0.0016 to 0.009 atm; the anneal is carried outat a temperature in the range of 710 to 740° C. and an oxygen pressurein the range of 0.0006-0.005 atm; and the anneal is carried out at atemperature in the range of 690 to 710° C. and an oxygen pressure in therange of 0.0003-0.002 atm.

[0050] In yet other preferred embodiments, sample is annealed atprogressively lower temperature and oxygen pressure. This may beaccomplished by continuously reducing temperature and/or oxygen pressureor by stepwise reduction of temperature and/or oxygen pressure.

[0051] Yet another aspect of the invention includes a superconductorhaving the formula Bi_(2−y)Pb_(y)Sr₂Ca₂Cu₃O_(10+x), where 0≦x≦1.5 andwhere 0≦y≦0.6, the oxide superconductor characterized by a criticaltransition temperature of greater than 111.0 K as defined by zeroresistance by a four point linear probe method with zero resistancecorresponding to a resistivity of less than 10⁻⁸ Ω-cm. An articlecontaining the oxide superconductor may additionally include silver.

[0052] Yet another aspect of the invention includes an oxidesuperconductor article characterized by a critical transitiontemperature of greater than 111.0 K as defined by zero resistance by afour point linear probe method with zero resistance corresponding to aresistivity of less than 10⁻⁸ Ω-cm and an x-ray diffraction patternhaving peaks at 17.4°, 19.2°, 20.2°, 21.8°, 23.2°, 23.9°, 26.2°, 27.8°,29°, 29.7°, 31.5°, 32°, 33.2°, 33.7°, 35°, 35.6°, 38°, 38.8°, 41.6°,43.8°, 44.4°, 46.8°, 47.4°, 48° and 49°.

[0053] Yet another aspect of the present invention provides for amultifilamentary oxide superconductor composite containing a pluralityof oxide superconductor filaments contained within a matrix materialwhich has been subjected to an anneal according to the presentinvention.

[0054] The oxide superconductor prepared according to the method of thisinvention exhibit superior electric transport properties and enhancedT_(c.)

BRIEF DESCRIPTION OF THE DRAWING

[0055] In the Drawing;

[0056]FIG. 1 is a processing profile of the two-step heat treatment ofthe invention; and

[0057]FIG. 2 is an optical photomicrograph of (a) a pressed oxidesuperconductor article without the two step heat treatment of theinvention, and (b) a pressed oxide superconductor article subjected tothe heat treatment of the invention;

[0058]FIG. 3 is a processing profile used to obtain an textured oxidesuperconductor according to the method including the two-step heattreatment of the invention;

[0059]FIG. 4 is a prior art Van't Hoff diagram for BSCCO-2223;

[0060]FIG. 5 is a phase stability limit diagram for BSCCO-2223indicating the low pressure, low temperature anneal conditions of thepresent invention;

[0061]FIG. 6 is a phase stability limit diagram for BSCCO-2223indicating the low pressure, low temperature anneal conditions of thepresent invention;

[0062]FIG. 7 is a plot of the critical transition temperature andcritical current v. anneal temperature for an article subjected to thetwo-step heat treatment and low pressure, low temperature anneal of theinvention; and

[0063]FIG. 8 is an x-ray diffractogram of an article subjected to thetwo-step heat treatment and low pressure, low temperature anneal of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT I. Two-Step Heat Treatment

[0064] The present invention is a method for improving the criticalcurrent density of oxide superconductor articles by healing defects,such as micro- and macrocracks, incurred upon deformation. The presentinvention calls for a final two-step treatment after deformation of theoxide superconductor article, in which (a) a liquid phase is formed suchthat the liquid phase co-exists with the desired oxide superconductor;and (b) the liquid phase is then transformed into the desired oxidesuperconductor without any intermediate deformation. The methods of theinvention can be used to heal defects in any oxide superconductor orsuperconducting composite as long as a liquid phase can co-exist withthe desired oxide superconductor phase. It is recognized, that completetransformation of the liquid to the oxide superconductor occurs underideal conditions and that, under some circumstances, not all of theliquid may be transformed into the desired oxide superconductor.

[0065] The liquid phase is formed upon partial melting of the oxidesuperconductor article. During partial melting of the article,non-superconducting materials and intermediate oxide phases may bepresent with the desired oxide superconductor phase. During the partialmelting step of the invention the desired oxide superconductor, thenon-superconducting materials, oxide superconducting precursors, thedesired oxide superconductor or a mixture of these components may meltto form the liquid phase.

[0066] The above process, which required that liquid co-exist with thedesired oxide superconductor phase, is distinguished from those whichinvolve the peritectic decomposition of the oxide superconductor, suchas described by Guo et al. and Kase et al., in which the desired oxidesuperconductor decomposes during the melting process.

[0067]FIG. 1 shows a processing profile of the final heat treatment ofthe invention. A dashed line 10 indicates a processing point at which aliquid phase is formed for a given set of processing conditions, e.g.,T, P_(O2), P_(total) and/or oxide composition.

[0068] In the oxide superconductors and superconducting compositesdisclosed herein, processing conditions for obtaining the requisiteliquid and solid oxide phases are well established and the relationshipbetween temperature, oxygen partial pressure and total pressure isreasonably well understood. For further information on the phasediagrams for YBCO, BSCCO and the thallium-based systems, the interestedreader is directed to “Phase Diagrams for High T_(c) Superconductors”,John D. Whitler and Robert S. Roth, Ed.; American Ceramic Society,Westerville, Ohio.

[0069] Presence of a liquid phase can also be determined experimentallyby use of such conventional techniques as differential thermal analysis(DTA). In DTA, exothermic and endothermic reactions as a function oftemperature can be identified and attributed to various thermodynamicand chemical processes. It is possible to identify endothermic processescorresponding to partial melting; i.e. liquid phase formation.

[0070] It is desired that only a small amount of liquid be formed duringpartial melting. The reason for this is that, at the time that the finalheat treatment is applied, the article substantially is alreadytextured. Complete or significant liquid formation at this point wouldresult in loss of texture. Volume percent of the liquid phase istypically in the range of 0.1 to 30.

[0071] The oxide superconductor is deformed at a point 11 before thefinal heat treatment, at which time defects such as microcracks may beintroduced into the article. Suitable deformation can include swaging,extruding, drawing, pressing, hot and cold isostatic pressing, rolling,and forging of wires, tapes and a variety of shaped articles.

[0072] The method of the invention is particularly useful for oxidesuperconductor articles in which deformation processing introducesdefects perpendicular to the direction of current flow. In rolling, thedeformation is largely plane strain, that is, extension in the length ofthe article entirely compensates for the reduction in thicknessresulting in cracking in the transverse direction and perpendicular tothe current flow. FIG. 2a shows an optical photomicrograph of a pressedoxide superconductor filament without the final heat treatment of theinvention. Many fine cracks are visible and clearly disrupt thepercolative pathway for current flow. It is expected therefore, thatrolled samples will benefit to a greater extent from the final heattreatment of the invention.

[0073] Other deformation processes, such as pressing, results inreduction of the article thickness accommodated by lateral spread, i.e.,an increase in width. Cracks in this case form parallel to the currentflow. While defects of this type, can be healed by the final heattreatment of the invention, the improvement to the electrical propertiesmay not be as marked.

[0074] Referring again to FIG. 1, the processing conditions are adjustedto bring the article to point 12 where the article is partially meltedand a liquid phase co-exists with the desired oxide superconductorphase. The article is held at point 12 for a period of time during whichthe defect surfaces contained within the oxide superconductor are wet bythe newly-formed liquid. In the case of BSCCO-2223, a temperature of820-835° C. at 0.075 atm O₂ for 0.1-300 hours is sufficient. Theprocessing parameters are then adjusted to bring the oxidesuperconductor to point 13 where the liquid phase is consumed and thedesired oxide superconductor phase is formed from the melt. In the caseof BSCCO-2223, a temperature of 820-790° C. at 0.075 atm O₂ for 1 to 300hours is sufficient.

[0075] The final heat treatment includes heating the article at atemperature of substantially in the range of 815° C. to 860° C. for aperiod of time substantially in the range of 0.1 to 300 h at a P_(O2)substantially in the range of 0.001-1.0 atm. The processing temperaturewill vary dependent upon the oxygen pressure. Additionally, variationsin the chemical composition of the article will also affect selection oftemperature and pressure. In particular, it has been noted that additionof silver to the oxide composition lowers the temperature range forpartial melting, particularly at higher P_(O2) (0.1-1.0 atm).

[0076] Hence, the two-step heat treatment heals cracks and other defectswhile effecting the final conversion of the oxide phases to the desiredoxide superconductor phase. The formation of BSCCO-2223 from BSCCO-2212is kinetically enhanced by the presence of the liquid phase, in part,due to the enhanced diffusivity of the oxide superconductorconstituents. The partial melting during the final part of the processcan perform two tasks. Firstly, the cracks formed during the priordeformation steps are healed by rapid growth of the oxide superconductorgrains at the crack site. Secondly, the conversion rate of 2212 to 2223is greatly accelerated, allowing the formation of a microscopicallycrack-free, interconnected 2223 phase.

[0077] Various processing parameters can be controlled to obtain thenecessary partial melt and oxide reforming steps. For example, P_(O2)can be held constant and temperature can be raised to promote meltingand formation of the liquid phase and lowered to regenerate the desiredoxide superconductor. Alternatively, temperature can be held constant,and P_(O2) can be lowered to promote the partial melting of the oxidesuperconductor article and raised to reform the oxide superconductor.

[0078] The processing conditions can be changed rapidly from point 12 topoint 13 of the process (fast ramp rate). Alternatively, the oxidesuperconductor can be subjected to gradually changing conditions (oftemperature or pressure) between point 12 and point 13 of the processdesignated by the curve 14 in FIG. 1 (slow ramp rate). In anotheralternative embodiment, there need be no “hold” at 13. The processingconditions can be slowly ramped from the processing conditions definedat point 12 to the processing conditions defined for point 13. Thisprocess is illustrated by curve 15 in FIG. 1.

[0079] The method of forming textured oxide superconducting articles isdescribed with reference to oxides of the BSCCO family; however, this isin no way meant to limit the scope of the invention. The presentinvention can be practiced with any oxide superconductor system in whicha liquid phase co-exists with an oxide superconductor phase and which isamenable to deformation-induced texture.

[0080] Texture may be induced by reaction conditions and/or deformation.In reaction-induced texture, processing conditions are chosen tokinetically favor the anisotropic growth of oxide superconductor grains.Reaction-induced texture can occur in a solid phase system or,preferably, in a solid-plus-liquid phase system. The presence of aliquid phase enhances the kinetics of anisotropic grain growth, probablythrough increased rates of diffusion of the oxide components. Indeformation-induced texture, a strain is applied to the superconductingarticle to induce alignment of the oxide grains in the plane ordirection of elongation. Deformation-induced texture requires aspectedgrains or an anisotropic rheology in order to effect a preferentialalignment of the grains.

[0081]FIG. 3 shows a processing profile for a method of the inventionused to obtain highly textured oxide superconductor articles. Thegeneral process for the formation of a textured oxide superconductor mayconsist of three distinct steps.

[0082] In a first step, an oxide superconductor precursor is subjectedto one or more first heat treatment/deformation iterations, denoted bystep 20 and step 21, respectively, of FIG. 3. The oxide superconductorprecursor can be any combination of materials which will yield thedesired oxide superconductor upon reaction. In particular, it may be ametallic alloy containing the metallic constituents of the desired oxidesuperconductor and optionally containing silver. Alternatively, theconstituent simple metal oxides, mixed metal oxides, metal salts andeven intermediate oxide superconductors of the desired oxidesuperconductor may be used as a precursor. The precursor may optionallybe mixed with a matrix metal, such as silver, and/or may be sheathed ina matrix material in a powder-in-tube configuration.

[0083] The heat treatment 20 of the heat treatment/deformation iterationserves two purposes in the process. Firstly, the anneal is sufficient toform an oxide superconductor and results typically in a mixture ofsuperconducting and secondary phases. “Secondary phases” includesub-oxide or non-superconducting oxide species which require furtherprocessing to form an oxide superconductor phase. BSCCO-2212 is oftenthe intermediate oxide superconductor because it is readily texturedduring mechanical deformation. BSCCO-2223 is the typical desired oxidesuperconducting phase because of its high critical temperature.Secondly, the heat treatment promotes reaction-induced texture.

[0084] The deformation 21 of the article promotes deformation-inducedtexture. One or more iterations can be performed. FIG. 3 shows two firstheat treatment/deformation iterations, by way of example. If more thanone iteration is performed, both conversion to the superconducting phaseand development of texture can be done in incremental stages.

[0085] If the desired oxide superconductor is not formed in the firstheat treatment/deformation iterations, the second step of the processmay consist of one or more second heat treatment/deformation iterationsto form the desired oxide superconductor and to further texture theoxide superconductor phase. The article is heated in a step indicated by22 whereby the desired oxide superconductor is formed andreaction-induced texture can occur. Secondary phases react withBSCCO-2212 to form the desired oxide superconductor, BSCCO-2223. Thearticle is deformed in a subsequent step indicated by 23, wherebydeformation-induced texture can occur. If more than one iteration isused, only a portion of the intermediate oxide superconductor, need beconverted into the desired oxide superconductor with each iteration.Conditions known to form intermediate and desired oxide superconductorsare well known in the art. Suitable conditions are described inSandhage, et al. JOM, 21 (Mar. 1991), hereby incorporated by reference.

[0086] Practically, the incremental improvement in alignment for bothanneal/deformation cycles will decrease markedly after severaliterations, however, there is no theoretical limit to the number ofiterations that can be used. The strain introduced in the deformationstep can range up to 99%. The strains applied in each deformation/annealiteration may be constant or they may be changed for each subsequentiteration. It is particularly desirable in some embodiments, to usedecreasing strains with each subsequent iteration.

[0087] It is also possible to adjust the processing conditions topromote partial melting during the heating step 20 or 22 of the heattreatment/deformation iterations, indicated by step 24, to assist ingrain growth and enhance reaction kinetics (reaction-induced texture).Heating in the range of 820-835° C. in 0.075 atm O₂ and 1 atm totalpressure for 0.1 to 100 hours is typical for partial melting to occur.

[0088] The final part of the process consists of a two-step heattreatment in which (a) the article is processed to form a liquid phasewhich co-exists with the desired oxide superconductor followed by (b)processing of the article under conditions that favor the formation ofthe desired oxide superconductor phase. Thus, step (b) of the two-stepheat treatment is designed to promote conversion of the non-″desiredoxide superconductor into the desired oxide superconductor. This processhas been described in greater detail, above.

[0089] The oxide superconductors which make up the oxide superconductorarticles of the present invention are brittle and typically would notsurvive a mechanical deformation process, such as rolling or pressing.For this reason, the oxide superconductors of the present invention aretypically processed as a composite material including a malleable matrixmaterial. In particular, silver is preferred as the matrix materialbecause of its cost, nobility and malleability. The oxide superconductorcomposite may be processed in any shape, however, the form of wires,tapes, rings or coils are particularly preferred. The oxidesuperconductor may be encased in a silver sheath, in a version of thepowder-in-tube technology. The oxide superconductor can take the form ofmultiple filaments embedded within a silver matrix. For furtherinformation on superconducting tapes and wires; see, Sandhage et al.

II. Low Pressure. Low Temperature Anneal

[0090] The present invention may also call for a low temperature, lowpressure annealing treatment after formation of a BSCCO-2223 oxidesuperconductor phase. The anneal is carried out in a low temperature,low pressure region which is within the phase stability region forBSCCO-2223. The phase stability region is that range of processingconditions for which BSCCO-2223 is a thermodynamically stable phase.Both oxygen pressure and temperature are known to be important (but notexclusive) processing parameters in determining the phase stabilityrange of BSCCO-2223. The applicants have surprisingly discovered that ananneal of the BSCCO-2223 oxide superconductor according to the inventionprovides significant improvements in electrical properties, inparticular, T_(c), I_(c), and J_(c). Improvements of up to 10%, 20% 30%and even up to 50% in critical current upon annealing according to theinvention has been observed. Bulk zero resistivity above 111.0 K afteranneal also has been observed.

[0091] The low temperature, low pressure anneal of the present inventionis particularly directed to Bi_(2−y)Pb_(y)Sr₂Ca₂Cu₃O_(10+x), where0≦x≦1.5 and 0≦y≦0.6. As is well known in the art, stated stoichiometriesare all approximate. Intentional and unintentional variations incomposition are contemplated within the scope of the invention. It isalso well known in the art to partially substitute one or more of theelements making up the oxide superconductor. In a particular embodiment,lead is substituted for between zero and thirty percent of the bismuth.Such partial substitution of elements of the Bi₂Sr₂Ca₂Cu₃O_(10+x) oxidesuperconductor are contemplated within the scope of the invention andare considered represented within the notation “BSCCO-2223” used todescribe the composition.

[0092] The low pressure, low temperature anneal of the invention iseffective to improve T_(c) and J_(c) of any BSCCO-2223 articleregardless of the process used to form the oxide superconducting phase.The BSCCO-2223 phase can be prepared from any conventional method,including by way of example only, solid state reaction of metal oxides,reaction of metal-organic precursors, oxidation of metallic precursorsand thin film deposition and reaction processes. Additionally, thearticle can be textured by any conventional method to induce orientationof the oxide superconductor grains (texturing). By way of example only,the article may be processed by zone-refined melt growth techniques andmechanical deformation such as pressing and rolling, as describedhereinabove. The two-step heat treatment of the invention is preferablyused in conjunction with the low pressure, low temperature anneal toobtain superior results.

[0093] Additional superconducting and non-superconducting phases can beincluded in the article, so long as they do not interfere with thesuperconducting properties of BSCCO-2223. By way of example only, othersuperconducting phases may include BSCCO-2212. By way of example only,non-superconducting phases may include noble metals which add ductilityand formability to the superconducting article. Noble metals includesilver and gold; silver is a preferred noble metal. The article may be amultifilamentary oxide superconductor in a silver matrix as describedhereinabove.

[0094] The anneal of the invention is most effective when carried out asthe final processing step in the preparation of the oxidesuperconducting article. That is, the formation and texturing of theBSCCO-2223 phase should be optimized before annealing the article usingthe low pressure, low temperature anneal of the invention. The inventionrecognizes that additional post-processing thermal treatments may berequired in the preparation of a usable article (for example, insulationof the superconducting article). Those steps are to be consideredseparate from the preparation of the oxide superconductor, for which theannealing of the present invention is preferably the final processingstep.

[0095]FIG. 4 shows a prior art phase stability diagram for BSCCO-2223 asa function of oxygen pressure and temperature, which is taken from Rubinet al. (J. Appl. Phys. Let. 61(16), 1977 (1992)). The region 31 above aline 30 (indicated with closed and open circles) defines the temperatureand oxygen pressure conditions at which the BSCCO-2223 phase is stable.The region 32 below line 30 defines the temperature and oxygen pressuresat which the BSCCO-2223 phase is unstable. The majority of the prior artprocessing of BSCCO-2223 is carried out under conditions defined by thesmaller region 33 in the upper left hand corner of the diagram boundedby the line 30 and lines 34.

[0096] The applicants have surprisingly discovered that the criticalcurrent and critical transition temperature of the BSCCO-2223 oxidesuperconductor can be improved by annealing the BSCCO-2223 phase underconditions defined by a hatched region 40 in FIG. 5. The annealing rangelies above and roughly parallel to the published stability boundary;however, there also exists an upper boundary, above whichsuperconducting properties will diminish. This region is satisfied bythe following equations (for T (C°) and P_(O2) (atm)):

500° C.≦T≦787° C.  (1);

P _(O2)≧3.5×10¹⁰ exp(−32,000/(T+273)) is the lower bound; and  (2)

(3) P_(O2)≦1.1×10¹² exp(−32,000/(T+273)) is the upper bound.  (3)

[0097] The vertices of the parallelogram defining region 40 are definedby the following coordinates [T(C°), P_(O2)(atm)]: (787, 0.09); (787,0.003); (500, 1.13×10⁻⁶); and (500, 3.7×10⁻⁸), which represent thecorner bounds for temperature and pressure conditions. The anneal ispreferably applied under conditions close to, but not exceeding, theupper boundary of region 40. While approaching the upper bound, thetemperature is preferably as low as kinetically and practicallypossible.

[0098] With reference to the phase diagram of FIG. 6, the BSCCO-2223phase is most beneficially annealed under conditions defined by ahatched region 50. The annealing range lies parallel and closelyapproaches the upper boundary of the annealing conditions. This regionis satisfied by the following equations (for T (C°) and P_(O2) (atm)):

500° C.≦T≦760° C.  (1);

P _(O2)≧8.5×10¹⁰ exp(−32,000/(T+273)) is the lower bound; and  (2)

P_(O2)≦2.62×10¹¹ exp(−32,000/(T+273)) is the upper bound.  (3)

[0099] The vertices of the parallelogram defining region 50 are definedby the following coordinates [T(C°), P_(O2)(atm)]: (760, 0.0092); (760,0.003); (500, 2.8×10⁻⁷); and (500, 9.0×10⁻⁸), which represent the cornerbounds for temperature and pressure conditions for region 50. Theconditions of regions 50 are particularly well-suited to improvecritical current and critical transition temperature of the oxidesuperconductor.

[0100] The oxygen pressures of the invention are carefully controlled bymaintaining a total annealing atmosphere at substantially oneatmosphere. The annealing atmosphere includes an inert gas such as argonor nitrogen. The inert gas is mixed with a precisely controlled amountof oxygen to obtain the desired total oxygen pressure. This gas iscontinuously introduced above the oxide superconductor article duringanneal. Hence, the annealing atmosphere remains essentially constant,since any changes in oxygen composition due to outgassing from thesample are mitigated by the flow of annealing atmosphere over thesample.

[0101] The oxide superconductor is annealed for a time sufficient toeffect the improvements in electrical properties of the presentinvention. The annealing time is a function of the anneal temperatureand the size of the superconducting article. Annealing time increases astemperature decreases and hence poses a limit to anneal temperatureswhich can be practically applied. At 660° C., greater than 150 hoursanneal time are required. If the superconducting article is sufficientlythin, then the annealing time decreases and the anneal may bepractically applied for temperatures as low as 500° C. Anneal timestypically range from 5 to 300 hours.

[0102] In preferred embodiments, the temperature is in the range of770-787° C. and the oxygen pressure is in the range of 0.017-0.085 atm;or the temperature is in the range of 750-770° C. and the oxygenpressure is in the range of 0.0009-0.052 atm; or the temperature is inthe range of 730-750° C. and the oxygen pressure is in the range of0.005-0.029 atm; or the temperature is in the range of 690-730° C. andthe oxygen pressure is in the range of 0.0001-0.015 atm. In morepreferred embodiments, the temperature is in the range of 740-760° C.and the oxygen pressure is in the range of 0.0016-0.009 atm; or thetemperature is in the range of 710-740° C. and the oxygen pressure is inthe range of 0.0006-0.005 atm; or the temperature is in the range of690-710° C. and the oxygen pressure is in the range of 0.0003-0.002 atm.

[0103] The annealing step can take the form of a single anneal attemperatures and oxygen pressures within the annealing regions definedby region 40 and 50 in FIGS. 5 and 6, respectively. Additional benefitsto the electrical properties of the BSCCO-2223 article are observed whenthe annealing step includes application of progressively lowertemperatures and/or oxygen pressures. In one preferred embodiment, theanneal may comprise two or more discrete “bakes” at progressively lowertemperatures and oxygen pressures. In another preferred embodiment, theanneal may comprise continuous slow temperature reduction of thesuperconducting article while oxygen pressure is either continuously orstepwise decreased. In yet another preferred embodiment, the anneal mayalso comprise continuous decrease of oxygen pressure while thetemperature is stepwise decreased. A particularly preferred embodimentincludes treating the oxide superconductor by a stepwise or continuousreduction in temperature and oxygen pressure from 787° C. at 0.075 atmoxygen to temperatures in the range of 730° C. to 690° C. at 0.003 to0.0003 atm oxygen. A prior anneal at 787° C. and 0.075 atm oxygen ispreferably carried out. At all times, however, the anneal must remainwithin the proscribed regions of either 40 or 50.

[0104] The method of approaching the low temperature, low oxygenpressure conditions of the anneal of the present invention is also afactor in the process. In preferred embodiments, the decrease intemperature and oxygen pressure are synchronized and gradual. An oxidesuperconductor article which has been formed at higher temperatures andoxygen pressures (as is typically the case) will retain an internaloxygen potential at the elevate prior processing levels until diffusionpermits the internal oxygen content to accommodate the external change.If the article is rapidly subjected to low temperature, low P_(O2)conditions, the BSCCO-2223 oxide superconductor may undergo irreversibledecomposition because the internal high oxygen potential of the articleresults in internal reaction conditions that are outside the upperboundary for oxygen pressure as defined by the present invention. Byslowly changing the temperature and oxygen pressure conditions, theinternal conditions are allowed to adapt to the changing externalconditions and remain within the anneal region of the present invention.

[0105] It is not certain what is the source of the improved criticalcurrent performance in the low pressure, low temperature anneal of thepresent invention. It has been suggested that oxygen stoichiometry isoptimized by the long anneal times at low oxygen pressure andtemperature. Alternatively, is may be that grain growth is optimized andthe number of grain boundaries is reduced, thereby facilitatingunimpeded current flow. An interesting possibility is the formation of anew oxide superconductor phase by atomic diffusion into the solid statestructure of BSCCO-2223.

[0106] The annealing of the present invention has been found to beparticularly effective when used in conjunction with the two-step finalheat treatment described above. The two-step heat treatment forBSCCO-2223 includes heating the article at a temperature substantiallyin the range of 815° C. to 860° C. for a period of time substantially inthe range of 0.1 to 300 h at a P_(O2) substantially in the range of0.001-1.0 atm and followed by reducing the article to a temperaturesubstantially in the range of 790° C. to 845° C. for a period of timesubstantially in the range of 1 to 300 hours at a P_(O2) substantiallyin the range of 0.01 to 1.0 atm. The reader is referred to “I. Two-StepHeat Treatment” of the Description of the Preferred Embodiment, above,for further discussion of the two-step heat treatment.

III. Examples of the Preferred Embodiment

[0107] The examples below describe the two-step heat treatment and theanneal step of the present invention. The anneal is used in conjunctionwith the two-step heat treatment; however, it can be used on BSCCO-2223oxide superconductor prepared by any conventional process. The J_(c)obtained from samples subjected to the two-step heat treatment showsignificant improvements over prior art materials. Even furtherimprovements in J_(c) are observed by subjecting an oxide superconductorarticle obtained from the two-step heat treatment process to a lowpressure, low temperature anneal.

EXAMPLE 1

[0108] The following example compares the transport critical currentcharacteristics of a samples treated with the two-step heat treatment ofthe present invention to those of conventionally processed samples.

[0109] Precursor powders were prepared from the solid state reaction offreeze-dried precursor of the appropriate metal nitrates having thenominal composition of 1.7:0.3:1.9:2.0:3.1 (Bi:Pb:Sr:Ca:Cu). Bi₂O₃,CaCO₃, SrCO₃, Pb₃O₄ and CuO powders could be equally used. Afterthoroughly mixing the powders in the appropriate ratio, a multisteptreatment (typically, 3-4 steps) of calcination (800° C.±10° C., for atotal of 15 h) and intermediate grinding was performed in order toremove residual carbon, homogenize the material and to generate the lowT_(c) BSCCO-2212 oxide superconductor phase. The powders were packedinto silver sheaths having an inner diameter of 0.625″ (1.5875 cm) and alength of 5.5″ (13.97 cm) and a wall thickness of 0.150″ (0.38 cm) toform a billet.

[0110] The billets were extruded to a diameter of ¼″ (0.63 cm). Thebillet diameter was narrowed with multiple die passes, with a final passdrawn through a 0.070″ (0.178 cm) hexagonally shaped die intosilver/oxide superconductor hexagonal wires. Nineteen of the wires werebundled together and drawn through a 0.070″ (0.178 cm) round die to forma multifilamentary round wire. The round wire was rolled to form a0.009″×0.100″ (0.023 cm×0.24 cm) multifilamentary tape.

[0111] A length of the composite tape was then subjected to a heattreatment according to the invention. The composite tape was heated in afurnace in a first heat treatment at 820° C. in 0.075 atm O₂ for 48 h.The first heat treatment formed significant amounts of the desired oxidesuperconductor phase, BSCCO-2223. The composite tape was then rolled toreduce thickness by 11% (0.009″ to 0.008″). Lastly, the rolled compositetape was subjected to a two-step heat treatment, namely, heating fromroom temperature at a rate of 1° C./min to 820° C. in 0.075 atm O₂ andholding for 54 h, cooling to 810° C. in 0.075 atm O₂ and holding for 30h. The sample was furnace cooled to room temperature in 1 atm P_(O2).

[0112] A comparable length of composite tape was subjected to aconventional heat treatment. The composite tape was heated in a furnacein a first heat treatment at 820° C. in 0.075 atm O₂ for 48 h. The firstheat treatment caused significant amounts of the desired oxidesuperconductor phase, BSCCO-2223 to form. The composite tape was thenrolled to reduce thickness by 11% (0.009″ to 0.008″). The controlsamples were then subjected to a second heat treatment at 810° C. in0.075 atm O₂ for 84 h. This was a single step heat treatment in which nomelting of the sample occurs. The microstructure of the samples wereevaluated under an optical microscope. The samples prepared according tothe method of the invention had a higher density and far less cracksthan the control samples.

[0113] The critical currents of the samples using a criterion of 1μV/cm, 77 K and zero applied field were determined. A single criticalcurrent was determined end-to-end over a long length of tape (7-10 m).Critical current for a number of 10 cm lengths of composite tapes weredetermined and an average value was determined. Critical current (I_(c)in A) is related to the critical current density (J_(c) in A/cm²) by therelationship, J_(c)≈1,250·I_(c). The results are reported in Table 1 andshow that samples processed according to the method of the inventionexhibited a factor of at least two improvement in critical transportproperties. TABLE 1 A comparative study of the method of the inventionwith a conventional process. sample no. length (m) I_(c) (A) % σ J_(c)(A/cm²) Example 1-1 10 6.05 — 7563 Example 1-2 0.1 9.52 13 11,900Control 1-1 7 2.23 — 2788 Control 1-2 0.1 4.08 16 5100

EXAMPLE 2

[0114] This example demonstrates that silver alloys can be used in placeof silver for the silver billet without detrimental effect on theelectrical properties of the composite. A composite tape is prepared asdescribed in Example 1; however, a silver alloy containing low levels ofMg and Ni was used to sheathe the oxide superconductor. The tape wasprocessed as in Example 1 according to the method of the invention. Theaverage I_(c) (77K, 10 cm) was 7.68 A as compared to ca, 4.08 A for aconventional process.

EXAMPLE 3

[0115] This example compares samples which have been pressed or rolledas the intermediate deformation.

[0116] Composite tapes were prepared as described in Example 1. Astatistically designed experiment was performed using the followingprocess parameters for the first anneal and final heat treatments.two-step heat treatment first heat treatment high temperature lowtemperature T(°C.) t(h) T(°C.) t(h) T(°C.) t(h) − 820 12 − 820 12 − 81036 0 827 24 0 827 24 0 815 54 + 835 48 + 835 48 + 815 72

[0117] Up to sixty four statistically selected combinations of reactionconditions were run both with and without the two-step heat treatment ofthe invention. The intermediate deformation step between the first heattreatment and two-step heat treatment consisted of a pressing with 12%strain reduction.

[0118] Comparable statistically designed experiments were carried outfor rolled samples with and without the two-step heat treatment of theinvention. The intermediate deformation step between the first andtwo-step heat treatment consisted of rolling with 12% strain reduction.Critical currents (77 K, 0T) were measured across 1 cm. The results arereported in Table 2. TABLE 2 Comparison of pressed and rolled compositetapes. deformation second heat sample no. treatment treatment n I_(c)(σ) (A) J_(c) (A/cm²) 3-1 press two step 60 10.63 (2.43) 13,288 3-2press one step 6  9.45 (0.20) 11,812 3-3 roll two step 60 11.71 (1.09)14,638 3-4 roll one step 64  3.41 (0.77) 4,263

[0119] The results found in Table 2 show that both pressed and rolledsamples benefitted from the two-step heat treatment of the invention.The benefit is greater for rolled samples because rolling results inmicrocracking perpendicular to the direction of current flow, which isthe most deleterious to transport critical current and the mostresponsive to the healing effect of the present invention.

EXAMPLE 4

[0120] This example shows the effect of the number of heattreatment/deformation iterations on critical transport properties of thesamples.

[0121] Ten meter lengths of composite tape were prepared as describedfor Example 1. The composite tapes were heated in a furnace in a firstheat treatment at 815° C. in 0.075 atm O₂ for 48 h. The composite tapeswere then rolled to reduce thickness by 12%. The above heat treat anddeform iteration was carried out for x=2, x=3 and x=4 iterations onthree samples, respectively. Lastly, the deformed composite tapes weresubjected to a two-step heat treatment according to the invention,namely, heating at 824° C. in 0.075 atm O₂ for 54 h, followed by heatingat 815° C. in 0.075 atm O₂ for 30 h. The final thickness for each tapewas 0.0080″ (0.020 cm). The critical transport measurements for thethree samples are given in Table 3. All samples exhibited criticalcurrents higher than that of the control sample (3 A). In this series ofsamples, the incremental improvement to critical current was maximizedat n=3; however, dependent upon the particular experimental conditions,it may be desirable to perform more or less iterations. TABLE 3 Effectof Iteration Number on Critical Transport sample no. x I_(c) (A) J_(c)(A/cm²) 4-1 2 6.35 7938 4-2 3 7.83 9788 4-3 4 6.98 8725

EXAMPLE 5

[0122] The affect of precursor powder stoichiometry was investigated.Composite tapes were prepared as described in Example 1, with thefollowing exception. Powders of different stoichiometry were used in thepreparation the composite tapes. Powder A: 1.8:0.4:2.0:2.2:3Bi:Pb:Sr:Ca:Cu Powder B: 1.7:0.3:1.9:2.0:3.1 Bi:Pb:Sr:Ca:Cu

[0123] The composite tapes prepared from powders A and B were subjectedto the following heat treatment/deformation cycle: (1) heat treatment:815° C., 0.075 atm O₂, 48 h (2) deformation: roll, 12% strain (3) heattreatment: 815° C., 0.075 atm O₂, 48 h (4) deformation: roll, 12% strain(5) two step heat treatment as described in Example 4

[0124] The final thickness of the tapes was 0.008″ (0.020 cm). Criticalcurrent measurements are reported in Table 4. TABLE 4 Effect of samplecomposition on critical current. 60 m length 10 cm lengths sample no.powder I_(c) (A) J_(c) (A/cm²) I_(c) (σ) (A) J_(c) (A/cm²) 5-1 A 8.447,174 11.8 (1.3) 10,030 5-2 B 10.4 8,840 11.1 (1.2) 9,435

EXAMPLE 6

[0125] A composite tape was prepared according to the processeddescribed in Example 1. The composite tape having a length of 44 m wassubjected to an heat treatment/deformation iteration comprised ofheating at 815° C. for 48 h under 0.075 atm O₂ and an 18% rollingdeformation. The iteration was repeated three times. The two-step heattreatment consisted of heating at 824° C. for 96 h (0.075 P_(O2)),followed by heating at 815° C. for 30 h (0.075 P_(O2)). The sample had amaterials J_(C) measurement of greater than 17,000 A/cm² (77 K, selffield).

EXAMPLE 7

[0126] This example illustrates the improved critical current forarticles prepared according to the low pressure, low temperature annealof the present invention.

[0127] A BSCCO-2223 oxide superconducting article is prepared using the“metallic precursor” process (MP). Bi—Pb—Ca—Sr—Cu—Ag alloys are made bymechanical alloying of the metallic elements in stoichiometricproportions of 1.84:0.34:1.85:2.01:3.5 Bi:Pb:Sr:Ca:Cu with 64 wt %silver. The precursor metallic alloy is fabricated into a multifilamentmetallic precursor-silver composites tape as described in Example 1,above. The tape contains 361 precursor filaments in a silver matrix. Thecomposite tape is oxidized in oxygen at approx. 400° C. for 150 to 400hours to convert the metallic precursor into simple and complex metaloxides (“suboxides”). The suboxides are then reacted at 760 to 800° C.(0.075 atm P_(O2), 1-20 hr) to form BSCCO-2212 and remaining reactantsnecessary to form BSCCO-2223. Deformation by rolling having a totalstrain in the range of 60 to 90% are used to texture the BSCCO-2212phase, as described in Example 1, above. BSCCO-2212 is converted toBSCCO-2223 as described above and in Example 1. Specifically, thecomposite tape is heated at 830° C. for 40 hours in air, followed byroll deformation at ambient to approx. 16% strain, followed by thetwo-step heat treatment of heating at 830° C. in 0.075 atm oxygen(balance argon for a total pressure of one atmosphere) for 40 hours andheating at 811° C. for 120 hours.

[0128] Other samples may be prepared using the oxide powder in tubemethod (OPIT), according to Example 1.

[0129] After formation of the BSCCO-2223 phase according to the abovemethods (or other conventional methods), the temperature is lowered at4° C./min to 787° C. Oxygen pressure is maintained at 0.075 atm andtotal pressure is one atmosphere (balance of atmosphere is argon). Theoxide superconductor composite is heated at 787° C. for 30 hours. Thefurnace atmosphere is maintained at the desired composition by carefullymixing argon with the proper amount of oxygen to attain an oxygenconcentration of 0.075 atm. The gas mixture flows through the furnaceand over the sample during the heat treatment.

[0130] Critical current is measure as described in Example 1 across a 1cm length. Critical current (I_(c)) increased from a 4.5 A pre-annealvalue to a 5.5 A post-anneal value for a single sample. (Criticalcurrent can be readily converted to critical current density by divisionor the area, which is typically approximately a factor of 0.00077 cm.

EXAMPLE 8

[0131] Example 8 shows the additional benefit to electrical propertieswhen an article is subjected to anneal at successively lowertemperatures and oxygen pressures.

[0132] An oxide superconductor composite is prepared according toExample 7, including a low pressure, low temperature anneal at 787° C.and 0.075 atm O₂. The oxide superconductor composite is then subjectedto an additional low temperature, low P_(O2) treatment. In a typicalprocedure, the sample is reduced in temperature at 2° C./min from 787°C. to the target lower temperature, which ranges as low as 724° C. inthis example. The furnace atmosphere is maintained at the desiredcomposition by carefully mixing argon with the proper amount of oxygen.The gas mixture is introduced into the furnace and passes over thesample during the heat treatment. The gas composition is adjusted withtime such that oxygen pressure is approx. 0.01 atm at the start of theperiod and 0.0035 atm as the target lower temperature is attained. Theconditions are then held for 45 hours, followed by cool down at 10°C./min to ambient P_(O2)=0.003).

[0133]FIG. 7(a) illustrates the improvement in critical current asanneal temperature is decreased. The improvement is shown as the percentincrease of critical current density over the critical current observedafter the first anneal at 787° C. (0.075 atm O₂) shown in Example 7. Itis understood that the sample from Example 7 is already an improvementover a conventionally processed material and even over materialsprocessed according to Examples 1-6. Increases of greater than 30% overthe critical current observed after the first anneal at 787° C. (0.075atm O₂) shown in Example 7 were observed. The sample shows a steadyimprovement in critical current density up to 738° C. (ΔJ_(c)=32%),after which the improvement is somewhat lessened. This may represent akinetic effect of annealing at reduced temperatures. It is expected thatimproved J_(c) may be obtained at even lower temperatures with longeranneal times. It is further expected that further reduction intemperature and oxygen pressure (under appropriate reaction times) willresult in even greater improvements in critical current density.

[0134]FIG. 7(b) illustrates the improvement in critical transitiontemperature as anneal temperature is decreased. T_(c,zero) as high a111.1 K were obtained for materials which were annealed at 745-752° C.Critical transition temperature is measured by the standard four probemethod which involves placing two voltage taps between two current taps,followed by passing a current through the sample and recording thevoltage with changing temperature. The sample is considered to besuperconducting with effectively zero resistance when resistance is lessthan 10⁻⁸ Ω-cm. The data presented in FIG. 7(b) represents zeroresistance measurements.

[0135] X-ray diffraction analysis of a powdered sample obtainedaccording to the method of Example 8, exhibited a unique diffractionpattern. In particular, the diffraction pattern (shown in FIG. 8)contained peaks at 2θ values of 17.4°, 19.2°, 20.2°, 21.8°, 23.2°,23.9°, 26.2°, 27.8°, 29°, 29.7°, 31.5°, 32°, 33.2°, 33.7°, 35°, 35.6°,38°, 38.8°, 41.6°, 43.8°, 44.4°, 46.8°, 47.4°, 48° and 49°. The peaks onthe diffraction pattern of FIG. 8 represent the peaks typicallyassociated with BSCCO-2223 and additional unidentified peaks. The peaksmost typically associated with BSCCO-2223 are 19.2°, 20.2°, 21.8°,23.2°, 23.96°, 26.2°, 29°, 31.5°, 32°, 33.2°, 33.7°, 35°, 35.6°, 38.8°,44.4°, 47.4°, 48° and 49°. These new peaks suggest the formation of oneor more new oxide superconductor phases.

[0136] As can be seen by the above examples, the method of the inventionis highly versatile and can be successfully used with a variety ofdeformation processes, oxide superconductor compositions, silver alloycompositions and processing conditions.

[0137] Other embodiments of the invention will be apparent to thoseskilled in the art from a consideration of the specification or practiceof the invention disclosed herein. It is intended that the specificationand examples be considered as exemplary only, with the true scope andspirit of the invention being indicated by the following claims.

What is claimed is:
 1. A method for preparing a BSCCO-2223 oxidesuperconducting article, comprising the steps of: providing an oxidesuperconductor article comprised of BSCCO-2223 oxide superconductor; andannealing the superconducting article at a temperature selected from therange of about 500° C. ≦T≦787° C. and an annealing atmosphere having anoxygen pressure selected from within the region having a lower bounddefined by the equation, P_(O2)(lower)≧3.5×10¹⁰exp(−32,000/T+273) and anupper bound defined by the equation,P_(O2)(upper)≦1.1×10¹²exp(−32,000/T+273), for a time sufficient toprovide at least a 10% increase in critical current density as comparedto the critical current density of the pre-anneal oxide superconductorarticle.
 2. A method for preparing a BSCCO-2223 oxide superconductingarticle, comprising the steps of: providing an oxide superconductorarticle comprised of BSCCO-2223 oxide superconductor; and annealing thesuperconducting article at a temperature selected from the range ofabout 500° C. ≦T≦760° C. and an annealing atmosphere having an oxygenpressure selected from within the region having a lower bound defined bythe equation, P_(O2)(lower)≧8.5×10¹⁰exp(−32,000/T+273) and an upperbound defined by the equation,P_(O2)(upper)≦2.62×10¹¹exp(−32,000/T+273), for a time sufficient toprovide at least a 10% increase in critical current density as comparedto the critical current density of the pre-anneal oxide superconductorarticle.
 3. A method for preparing a BSCCO-2223 oxide superconductorarticle, comprising the steps of: exposing an oxide superconductorarticle to a heat treatment after deformation of the article, thearticle comprised of at least BSCCO-2223, the heat treatment comprising,(a) heating the article at a temperature substantially in the range of815-860° C. for a period of time substantially in the range of 0.1 to300 hours at a P_(O2) substantially in the range of 0.001-1.0 atm; (b)subjecting the article to a temperature substantially in the range of790-845° C. for a period of time substantially in the range of 1 to 300hours at a P_(O2) substantially in the range of 0.01-1.0 atm, with nodeformation occurring after the heat treatment; and annealing thesuperconducting article at a temperature selected from the range ofabout 500° C. ≦T≦787° C. and an annealing atmosphere having an oxygenpressure selected from within the region having a lower bound defined bythe equation, P_(O2)(lower) ≧3.5×10¹⁰exp(−32,000/T+273) and an upperbound defined by the equation, P₀₂(upper) <1.1×10¹²exp(−32,000T+273). 4.The method of claim 3, wherein the step of annealing the superconductorarticle comprises annealing at a temperature selected from the range ofabout 500° C. ≦T≦760° C. and an annealing atmosphere having an oxygenpressure selected from within the region having a lower bound defined bythe equation, P_(O2)(lower)≧8.5×10¹⁰exp(−32,000/T+273) and an upperbound defined by the equation, P_(O2)(upper)≦2.62×10¹¹exp(−32,000/T+273).
 5. The method of claim 1 or 2, wherein thestep of annealing comprises annealing under temperature and oxygenpressure conditions sufficient to provide at least a 30% increase incritical current density as compared to the critical current density ofthe pre-anneal oxide superconductor article.
 6. The method of claim 1, 2or 3, wherein the annealing atmosphere is substantially at a pressure ofone atmosphere and oxygen pressure is obtained by controlling the oxygenconcentration in the annealing atmosphere.
 7. The method of claim 3,wherein annealing atmosphere additionally comprises an inert gasselected from the group consisting of argon, nitrogen, helium and neon.8. The method of claim 1, 2 or 3, wherein the step of annealing is thefinal annealing to which the superconducting article is subjected. 9.The method of claim 1, 2 or 3, wherein the anneal is carried out at anoxygen pressure substantially in the range of 7.5×10⁻² atm to 1×10⁻⁸ atmO₂.
 10. The method of claim 1, 2 or 3, wherein the anneal is carried outat a the temperature in the range of 770 to 787° C. and an oxygenpressure in the range of 0.017 to 0.085 atm.
 11. The method of claim 1,2 or 3, wherein the anneal is carried out at a temperature in the rangeof 750 to 770° C. and an oxygen pressure in the range of 0.0009 to 0.052atm.
 12. The method of claim 1, 2 or 3, wherein the anneal is carriedout at a temperature in the range of 730 to 750° C. and an oxygenpressure in the range of 0.005 to 0.029 atm.
 13. The method of claim 1,2 or 3, wherein the anneal is carried out at a temperature in the rangeof 690 to 730° C. and an oxygen pressure in the range of 0.0001-0.015atm.
 14. The method of claim 1, 2 or 3, wherein the anneal is carriedout at a temperature in the range of 740 to 760° C. and an oxygenpressure in the range of 0.0016 to 0.009 atm.
 15. The method of claim 1,2 or 3, wherein the anneal is carried out at a temperature in the rangeof 710 to 740° C. and an oxygen pressure in the range of 0.0006-0.005atm.
 16. The method of claim 1, 2 or 3, wherein the anneal is carriedout at a temperature in the range of 690 to 710° C. and an oxygenpressure in the range of 0.0003-0.002 atm.
 17. The method of claim 1, 2or 3, wherein the step of annealing comprises annealing at progressivelylower temperature and oxygen pressure.
 18. The method of claim 17,wherein the step of annealing comprises continuously reducingtemperature and/or oxygen pressure.
 19. The method of claim 17, whereinthe step of annealing comprises stepwise reduction of temperature and/oroxygen pressure.
 20. An oxide superconductor composite preparedaccording to any of claims 1, 2 or
 3. 21. An oxide superconductorarticle comprising an oxide superconductor having the formulaBi_(2−y)Pb_(y)Sr₂Ca₂Cu₃O_(10+x), where 0≦x≦1.5 and where 0≦y≦0.6, theoxide superconductor characterized by a critical transition temperatureof greater than 111.0 K as defined by zero resistance by a four pointlinear probe method with zero resistance corresponding to a resistivityof less than 10⁻⁸ Ω-cm.
 22. The oxide superconductor article of claim21, further comprising silver.
 23. The oxide superconductor article ofclaim 21, wherein the oxide superconductor comprises a filamentary oxidesuperconductor phase contained within a silver phase.
 24. An oxidesuperconductor article characterized by a critical transitiontemperature of greater than 111.0 K as defined by zero resistance by afour point linear probe method with zero resistance corresponding to aresistivity of less than 10⁻⁸ Ω-cm and an x-ray diffraction patternhaving peaks at 17.4°, 19.2°, 20.2°, 21.8°, 23.2°, 23.9°, 26.2°, 27.8°,29°, 29.7°, 31.5°, 32°, 33.2°, 33.7°, 35°, 35.6°, 38°, 38.8°, 41.6°,43.8°, 44.4°, 46.8°, 47.4°, 48° and 49°.
 25. An oxide superconductorarticle characterized by a critical transition temperature of greaterthan 111.0 K as defined by zero resistance by a four point linear probemethod with zero resistance corresponding to a resistivity of less than10⁻⁸ Ω-cm and an x-ray diffraction pattern having peaks at 17.4°, 27.8°,29.7°, 38°, 41.6° and 46.8°.